I think it is inconsiderate if you expect everyone to wait for you.
However, I often order a starter because I intend to have a starter and a pud, instead of 3 courses. I want my pud
and round here the portions are massive. The starters are honestly the size of an adult's meal. They aren't tiny little appetizers. Plus the choices are often nicer than the main.
I ask for my starter to be brought out at the same time as everyone's main. That way we all eat together, but I have the meal combination I want.
I would suggest you do that. Order the mains at the same time as the person orders their starter and tell the waiter/waitress to bring out the mains as soon as they're ready and not to wait for the starter to have been finished.
I suppose there will be those who say that's rude.